While PS gives more Chi for casting more spells and thus creating more Mana Tea charges, Ascension directly gives more mana and causes each Mana Tea stack to return more mana due to the bigger mana pool.Which do you consider better for PvE healing and why?

I went to Ascension immediately after it was changed. It didn't help with mana as much as I'd hoped. I changed to PS and to be honest, I feel like it has more longevity built in. The little bit of extra burst doesn't hurt either.

1 free chi per 20s = 3000 mana per 20s
PLUS The amount of mana you attribute to the Chi itself. The most simple approach is to give it the cost of one extra jab. Doing this means that the comparative gain from that talent goes up if you are on a SCK spam encounter and less if you are using SoM to pool Chi.
Jab costs 11700 mana
So total, PS is worth 14700 mana per 20s or 3675 mp5

Ascension grants 45k base mana +15% per tea charge
Now on sustained encounters, I tend to use the hypothesis of 2 tea charges consumed every 15s (garalon like) but let's do the math for 2 tea per 15s and 2 tea per 20s and compare

In order to fully compare though, you need to consider how much "regen" you can attribute to the bonus 45000 mana. This is tricky as it obviously highly varied on the duration of the encounter. The longer the encounter, the more marginal this is. Yet to show a point, let's say you compare it on a super short encounter say 3 mins (almost innexistant atm).
45000 mana over 3 min is equivalent to 1250 mp5

Thus the grand total with very bold hypothesis (aka highly favoring ascension) puts ascension at a total of
1200 mp5 + 1250 mp5 = 2450 mp5.

Tl;dr conclusion:
Power Strikes is significantly better in terms of net mana and has a slight HPS increase. The one reason to use Ascension is truly the niche of having a 5th chi. This helps create better aoe heal setups. Yet for big AoE encounters, Chi Brew allows better setups every other big AoE than Ascension does whilst providing better regen as well (it is competitive with PS if used on CD or almost but since that generally won't be the case, it varies by the players' playstyle)

As to what I go for - Lately I've been running ascension and my mana has become so frustrating (in ilvl 492) that I will very, very likely turn back to either the others. There are a few encounters where I still prefer standing at range for which I may still run ascension over Chi Brew but for any encounter with natural jabing, PS will be my pick.

You forgot to add in the passive 900 Mp5 associated with having 45k higher maximum mana, as per the base mana regeneration formula. It brings it from BS/Chi Brew blowing Ascension out of the water to "pretty damn close."

Of course with Jab still at 9,000 mana, Ascension beats it outright in terms of regen.